Quantitative measurement of quantum dot uptake at the cell population level using microfluidic evanescent-wave-based flow cytometry

The intracellular uptake of nanoparticles (NPs) is an important process for molecular and cellular labeling, drug/gene delivery and medical imaging. The vast majority of investigations into NP uptake have been conducted using confocal imaging that is limited to observation of a small number of cells. Such data may not yield quantitative information about the cell population due to the tiny sample size and the potential heterogeneity. Flow cytometry is the technique of choice for studying cell populations with single cell resolution. Unfortunately, classic flow cytometry detects fluorescence from whole cells and does not shed light on subcellular dynamics. In this report, we demonstrate the use of microfluidics-based total internal reflection fluorescence flow cytometry (TIRF-FC) for examining initial quantum dot (QD) entry into cells and the associated subcellular movement at the single cell level with a rate of ∼200 cells s^-1. Our cytometric tool allows extraction of quantitative data from a large cell population and reveals details about the QD transport in the periphery of the cell membrane (∼100 nm deep into the cytosol). Our data indicate that the fluorescence density at the membrane vicinity decreases after initial QD dosage due to the decline in the density of QDs in the evanescent field and the transport into the cytosol is very rapid.